Refractory cores

ABSTRACT

Refractory cores, preferably used in investment casting, are hardened and strengthened by impregnating them with a melt of at least one organic compound characterized by (1) a melting point of at least 170* F., (2) the capability of being melted to a liquid and of being resolidified upon cooling, (3) the capability of being volatilized when heated to a temperature above the melting point, and (4) a preferred density of at least one gram per milliliter. The organic materials used for impregnation are selected from the classes consisting of non-cyclic or open-chain hydroxyl containing compounds and cyclic compounds, i.e., cyclic aliphatic and substituted or unsubstituted aromatic compounds including arenes. Molds which contain the impregnated cores are heated prior to being cast with metal to a temperature sufficient to volatilize the compound.

United States Patent Horton 1 Sept. 5, 1972 [54] REFRACTORY CORES [72] Inventor: Robert A. Horton,

Ohio

[73] Assignee: Precision Metalsmiths, Inc.,

[22] Filed: Feb. 22, 1971 [21] Appl. No.: 117,751

Chesterland,

[52] US. Cl. ..l64/72, 164/121, 164/138, 164/369, 1l7/5.2 [51] Int. Cl ..B22c 3/00 [58] Field of Search ..164/72, 74, 121, 138, 369; 117/52, 54, 123

Primary Examiner-J. Spencer Overholser Assistant ExaminerJohn E. Roethel Att0meyWatts, Hoffman, Fisher & Heinke [57] ABSTRACT Refractory cores, preferably used in investment casting, are hardened and strengthened by impregnating them with a melt of at least one organic compound characterized by (1) a melting point of at least 170 F., (2) the capability of being melted to a liquid and of being resolidified upon cooling, (3) the capability of being volatilized when heated to a temperature above the melting point, and (4) a preferred density of at least one gram per milliliter. The organic materials used for impregnation are selected from the classes consisting of noncyclic or open-chain hydroxyl containing compounds and cyclic compounds, i.e., cyclic aliphatic and substituted or unsubstituted aromatic compounds including arenes. Molds which contain the impregnated cores are heated prior to being cast with metal to a temperature sufficient to volatilize the compound.

6 Claims, No Drawings REFRACTORY CORES BACKGROUND OF THE INVENTION The present invention relates generally to the preparation of refractory cores, and more specifically to the strengthening and hardening of refractory cores for use in metal casting processes, especially investment casting.

Preformed refractory cores are widely used in investment casting, and to a lesser extent in other casting processes, to form holes, slots, etc. in the castings. The cores are used to make investment molds in several ways. Often the core is positioned within the pattern material injection die and molten wax or other pattern material is injected around the core to form the pattern. Disposable patterns of wax or other material also are made separately and the cores inserted into openings in the patterns. In either case the cores become incorporated into the investment molds which are formed around the patterns. The core is made to extend beyond the pattern at one or more locations and becomes embedded in the mold at these locations so that it is firmly held in position when the pattern is melted out of the mold. In other casting procedures, the cores are assembled directly into molds, for example, molds of the cope and-drag type, which are provided with suitable core prints in which the cores are seated.

The use of refractory cores as generally described above can enlarge the scope of the casting process to permit the manufacture of parts that would not otherwise be feasible and can often reduce casting costs by lowering rejection rates and simplifying the molding operations. In spite of these advantages, the extensive use of refractory cores has been limited because of the difficulties involved in making cores which have the high strength and hardness necessary to resist breakage and/or damage during precasting operations and which do not exhibit excessive strength at the time of casting.

The size and shape of the cores are dictated by the desired configurations of the castings. Frequently, the cores are relatively thin and of a complex shape, and in such instances conventional ceramic cores are very fragile. Excessive breakage is encountered in normal handling, storage, shipping, and production operations, such as when disposable patterns are injected around the cores. The cores are often scratched or abraded in the same operations so as to destroy their surface finish. In many instances it is desired to perform machining operations on the cores, such as drilling holes, etc. The ability successfully to perform such machining operations is seriously impaired by the fragility of the cores.

While refractory cores must resist the thermal and mechanical stresses encountered during the casting of molds, the strength required for casting is generally less than that required to resist breakage in other operations. Cores which are subject to excessive breakage during handling, shipping, etc. may have sufficient strength for the actual casting process. It has in fact been found that excessive core strength at the time of casting should be avoided and that the cores should not be any stronger than necessary to resist the thermal and mechanical stresses which are encountered. Excessive strength of the cores can result in hot tearing of the castings and can make it difficult easily to remove the cores from the castings.

Various procedures have been followed in efforts to improve the strength and hardness of refractory cores so as to prevent them from being broken and/or damaged during handling, shipping, machining, etc. One such prior art practice is to fire the cores during their manufacture to higher temperatures than are needed to provide the strength necessary for casting. This practice is not desirable for a variety of reasons. It causes increased sintering shrinkage which makes it difficult to control dimensions and prevent warpage. It also reduces the porosity of the cores so that they are not readily permeable to the hot mold gases, and this results in gas entrapment in the castings. The excessive strength which is obtained by high temperature sintering is undesirable during the casting operation for the reasons mentioned above.

Another prior art practice is to add inorganic, low melting materials, such as feldspar, to the core batch. Although the addition of such materials lowers the sintering temperature, this practice has all of the disadvantages associated with high temperature sintering. In addition, the finished cores are rendered less refractory.

Refractory cores have also been impregnated with pattern waxes. This practice does not achieve the desired strength and hardness, and the cores are still subject to excessive breakage.

Still another prior art practice has been to impregnate refractory cores with various thermosetting resins. The resins and/or catalysts which have been used in this practice are often toxic or harmful to the skin. When room temperature curing catalysts are employed, the resin batch is unstable and must either be used within a limited time or discarded. High temperature curing catalysts may require extended oven curing to harden the resin. Cores which are placed into baskets for this purpose, as is desirable in mass production, may stick together. Still other disadvantages are encountered when investment molds containing cores impregnated with a thermosetting resin are fired to prepare the molds for casting. The higher thermal expansion of the resin compared to that of the mold and the core may cause disruption of the ceramic or refractory material. In addition, the thermosetting resins cannot be evaporated from the mold, but must be decomposed and the carbonaceous residue oxidized. This makes firing of the molds and burnout of-the mold cavities a difficult operation to accomplish.

SUMMARY OF THE INVENTION An object of the present invention is to provide refractory cores which have the physical properties required for their successful, economic use in metal casting processes, especially investment casting. A more specific object of the present invention is to provide refractory cores which possess the high strength and hardness necessary to resist breakage and/or damage during pre-casting operations, and which at the time of casting are permeable to the hot mold gases and do not possess excessive strength. 8

Another object of the present invention is to provide for the preparation of refractory cores characterized as described above using methods and materials which avoid the difficulties encountered with the prior art practices of strengthening ceramic cores.

In accordance with the invention, preformed refractory cores of the desired configuration which have been made using any conventional refractories and techniques are impregnated with a melt of at least one organic compound, characterized in part by a melting point of at least 170 F., the capability of being melted without excessive decomposition to a liquid having sufficient fluidity to penetrate the pores of the cores, and the capability of being resolidified upon cooling to a hard, preferably crystalline, state. The preferred materials used for impregnation are non-cyclic or open-chain hydroxyl containing compounds, especially solid acids and solid alcohols, and cyclic compounds, i.e., cyclic aliphatic and substituted and unsubstituted aromatic compounds including arenes. The impregnation of the cores with such materials can be expeditiously accomplished, as by dipping, and it is not necessary to apply a vacuum or pressure to the melt in order to obtain satisfactory penetration.

When the impregnated cores are cooled to room temperature, the preferred classes of organic materials produce enormous increases in strength and hardness. Exemplary cores prepared in accordance with the invention show an increase in modulus of rupture of up to 12 times in comparison to unimpregnated cores. The hardened and strengthened cores can be handled, stored, shipped, and used in the most expeditious manner in production without breakage and/or surface damage. The strength and hardness of the cores permits machining operations to be performed and enables the cores to be used in wax injection dies to make disposable patterns for investment casting. Because of the fact that the organic compounds used for impregnation have melting points of at least 170 F the strength of the cores is retained during wax injection operations which are typically performed at temperatures of from 140 to 160 F.

The invention makes it possible to eliminate the conventional high temperature sintering practice and the attendant disadvantages. Cores can be fired only to the minimum temperatures which are necessary to develop the strength required during casting, and then impregnated as described above to produce the higher strengths which are desired for pre-casting operations. In some instances, the need for sintering can be eliminated entirely. For example, the cores may be made with a binder which will provide the strength needed for casting. In other applications, unfired cores may be impregnated with an inorganic material, such as colloidal silica or hydrolized ethyl silicate, which will impart the strength and hardness necessary for casting, and then processed according to this invention in order to further increase the strength and hardness of the cores.

Investment molds which have been made using the impregnated cores can be processed in accordance with conventional techniques to melt out the disposable patterns and prepare the molds for casting. Since the preferred organic materials used for impregnation melt to a liquid upon heating without significant decomposition, the materials do not have a disruptive effect on the ceramics of the molds and cores during firing. Another characteristic of the preferred classes of impregnating materials is that they volatilize upon heating to a temperature above the melting point. This characteristic permits the materials to be readily evaporated from the molds when they are fired preparatory to casting. At the time of casting the cores will have the desired porosity and will have no more strength than is necessary to resist the thermal and mechanical stresses encountered during casting.

Other objects, advantages and a fuller understanding of the invention will be had from the following detailed description. 1

DESCRIPTION OF THE PREFERRED EMBODIMENTS The invention can be practiced in connection with preformed, porous refractory cores which have been produced using any desired refractory compositions and core-forming techniques, and is not limited or restricted to any particular refractories, refractory compositions and methods of manufacture. Typical refractory materials which are used to make cores include fused and crystalline silica, zircon, zirconia, aluminia, calcium zirconate, various aluminum silicates, and the like. ln accordance with conventional practices, batch mixtures consisting essentially of the selected refractories and suitable binders are molded to the desired shapes by such well-known techniques as injection molding, slip casting, dry pressing, transfer molding, and the like. The molded refractory cores may be fired and/or impregnated in order to obtain the strength desired for casting.

Pursuant to this invention, preformed refractory parts such as those made in accordance with the practices generally described above are impregnated with a melt of one or more organic compounds characterized by a melting point of at least F., the capability of being melted without significant or excessive decomposition to a liquid having sufficient fluidity to penetrate the pores of the cores, and by the capability of being resolidified upon cooling to a hard, preferably crystalline, state. The preferred organic compounds employed to strengthen and harden refractory cores for use in metal casting processes are further characterized by a density of at least one gram per milliliter and by the capability of being volatilized upon heating to a temperature above the melting point.

Cores impregnated with organic compounds having the foregoing characteristics exhibit an exceptional combination of properties which have not been obtained using other impregnating agents. Organic materials having melting points less than 170 F. and densities less than one gram per milliliter do not produce the high strengths and hardnesses which are obtained using the preferred compounds. The capability of the preferred materials of being melted to a liquid without excessive decomposition and of being resolidified upon cooling makes it possible to impregnate the cores expeditiously by dipping them into a hot melt of the compound or compounds. Such compounds have sufficient fluidity to penetrate the cores to the desired extent, and it is not necessary to apply a vacuum or pressure to the melt. Because of the fact that the preferred materials volatilize upon heating to a temperature above their melting points, as distinguished from materials which decompose, the impregnating agents can be readily evaporated from the molds when they are fired preparatory to casting without forming a carbonaceous residue in the mold cavities. Volatilization of the impregnating materials has the desired effects of restoring core porosity and reducing the core strength to that obtained by firing and/or impregnating the cores with other materials during their manufacture.

Organic compounds exhibiting the foregoing properties and used for impregnating cores in accordance with this invention are non-cyclic compounds, both branched and straight chain structures, which are characterized by the presence of the hydroxyl (OH) group, and cyclic compounds. The class of non-cyclic or open-chain hydroxyl containing compounds includes solid acids, preferably carboxylic acids and derivatives, such as diglycolic acid (2,2-oxydiacetic acid), adipic acid, azelaic acid, sebacic acid, glutaric acid, malonic acid, and the like; and solid alcohols and derivatives, such as trimethylolethane, pentaerythritol, erythritol, 2-amino-2-( hydroxymethyl)-l ,3 propanediol, dulcitol, mannitol, sorbitol, and the like.

The class of cyclic compound includes cyclic aliphatic compounds and substituted and unsubstituted aromatic compounds including arenes (substituted and unsubstituted compounds containing both aliphatic and aromatic units). Compounds of this class which have been used successfully include alcohols, phenols, ketones, imides, amides, ketoamides, anhydrides, aldehydes, nitro compounds and hydrocarbon compounds. Examples of useful cyclic aliphatic compounds are cholesterol (cholesterin; 5-cholesten-3 beta-o1), terpin hydrate, succinimide, succinic anhydride, and the like. Examples of useful aromatic compounds are catechol (l,2-dihydroxybenzene), resorcinol (1,3- dihydroxy benzene), hydroquinone, benzoin, benzil, phthalimide, benzamide, phthalic anhydride, terephthaldehyde, m-di-nitrobenzene, acetoacetanilide, anthracene, and the like.

Specific materials which have been used with particular success are the open-chain, aliphatic compounds which have four to six carbon atoms and three or more hydroxyl groups, and more especially are the straight chain polyhydric alcohols, and still more specifically are mannitol, dulcitol, sorbitol, and erythritol. Mannitol is an especially preferred material because it produces excellent results and is economic.

Several materials having the desired characteristics and falling within the foregoing classes of useful materials may be toxic and/or harmful to the skin, eyes or lungs. For example, catechol is considered toxic and inhalation or contact with the eyes or skin should be avoided. Technical grades of resorcinol contain a small amount of free phenol and may require similar precautions to assure safety. Such compounds, while satisfactory from a technical standpoint, are not recommended because of the special precautions which should be taken to assure safety during use. Straight chain polyhydric alcohols, such as mannitol, dulcitol, sorbitol, erythritol, and the like, produce excellent strength and hardness and are not generally considered to be toxic or to cause dermatitis.

The impregnating process is carried out by melting the selected organic material or materials to a fluid consistency. The materials preferably should not be heated to boiling or to a temperature which would cause excessive deterioration. It is preferable to heat the cores prior to being impregnated with the melted material, although this is not necessary. The cores are impregnated in any suitable manner, as by dipping. For example, the cores may be loaded into metal baskets having sufficiently large openings to permit easy draining and the baskets of cores lowered into the molten bath for the desired length of time. The basket of cores is then removed and may be allowed to drain over the bath in order to minimize drag-out losses of the organic 'material.

The length of time that the cores are allowed to remain in the heated bath of melted organic material, or the amount of organic material which is used to impregnate cores by other procedures, can be varied widely. The maximum increase in strength is obtained by allowing the impregnating material to fill all of the pores of the ceramic core. However, complete impregnation is not essential. In situations where increased surface hardness rather than strength is of primary importance, it is only necessary to allow the impregnating agent to penetrate about one thirty-second of an inch into the core. In general it is preferable to preheat the cores before impregnation if high strength is required. High strength can be obtained without preheating of the cores, but the time required for impregnation will be longer.

The presence of the impregnating agent on the surfaces of cores is not considered to be harmful, since the cores are subsequently heated and the impregnating agent volatilized or evaporated from the molds prior to casting. However, the appearance of the cores can be improved if the excess material remaining on the core surfaces after impregnation is removed. When cores are individually impregnated, as may be the case with large cores, the excess can often be removed simply by shaking the cores after they have been removed from the impregnating bath. When the cores are being handled in bulk, a rinsing procedure is generally used.

Rinsing is best accomplished in two stages. The first stage involves the application of a liquid in which the impregnating agent is soluble, and the second stage involves the use of a liquid which will dissolve or remove the first liquid but only has limited solubility for the impregnating agent. By way of example, many of the useful impregnating agents are soluble in hot water, but are only slightly soluble in alcohol. In such instances, hot water, preferably boiling water,.may be used for the first rinse step followed by isopropyl alcohol for the second. In the event that the organic compounds are too soluble in isopropyl alcohol to permit its use for the second rinse, the alcohol may be used for the first rinse and a liquid such as benzene or toluene used for the second rinse. The water-isopropyl alcohol sequence rinse is usually used with mannitol, a preferred impregnating agent.

The time in the first rinsing bath should be kept to a minimum in order to avoid leaching the impregnating agent from the core which can reduce surface hardness, and to avoid penetration of the rinse liquid into the core surface. These undesirable conditions are easily avoided, since only one or two quick rinses of an inand-out nature are generally sufficient. v

Refractory cores which have been strengthened and hardened by impregnation in accordance with this invention can be machined and handled in the most expeditious manner during the production of castings. In the case of investment casting, the cores can be positioned in the injection die of a wax injection machine, and molten wax or other pattern material injected around the core to form disposable patterns. The temperatures of wax injection are typically in the range of from about 140 to 160F. with the maximum temperature usually being about 170 F. Because of the relatively high melting point, i.e., at least 170 F., of the organic compounds used as impregnating agents, the strength of the impregnated cores will be retained during the injection process. It is also possible to use the impregnated cores in other ways, as by inserting the cores into openings of the patterns.

Investment molds containing the impregnated cores can be dewaxed by conventional procedures. For example, the molds may be placed in a furnace operating at an elevated temperature, for example, at a temperature in the range of from about l,600 F. to about l,800 F. Another dewaxing procedure is to expose the molds to an atmosphere of saturated steam under pressure in an auto-clave.

After the patterns have been removed from the investment molds, it is customary to fire the molds to an elevated temperature so that the molds are hot during the casting operation. The firing of the molds during the pattern removal operation and/or prior to casting is effective toevaporate the organic impregnating agents from the molds without leaving a carbonaceous residue in the mold cavities. Evaporation of the impregnating agent from the cores restores their porosity so that the cores are permeable to the hot mold gases, and reduces the core strength so that it is not excessive at the time of casting.

The invention is illustrated by the following examples. Unless otherwise specified, the percentages are given as percentages by weight.

EXAMPLE I Refractory cores were made by dry pressing the following composition:

8% low melting point paraffin, I

37% fused silica powder,

45% zircon powder( 45% plus 325 mesh) 10% zircon powder (95% minus 325mesh) A batch of the foregoing composition was prepared by first blending together the refractory powders and then warming the powders to 200 F. The paraffin was melted separately and all ingredients were combined using 'a mixer having a whip type agitator. The mix material was granulated by passing it while still warm through a US'No. 16 mesh screen, after which the granules were allowed to cool to room temperature. The granulated core batch was loaded into a steel die and pressure under 25 tons force to produce green cores 1 inch wide by 5 inches long and one-quarter inch in thickness.

The green cores were embedded in a coarse aluminum silicategrog and fired in a furnace at 2,300 F. for 4 hours. The furnace was then turned off and allowed to cool over night.

One-half of the cores werepre-heated to 400 F. and impregnated for three minutes in molten mannitol at the same temperature. After impregnation, the cores were drained briefly to permit excess mannitol to run back into the bath. The cores were then plunged while still hot into boiling water and agitated briefly. Following the water rinsing step, the cores were rinsed thoroughly in isopropyl alcohol after which they were dried by forced air.

The impregnated and the untreated cores were broken on a 4 and l l 6 inch span with center loading in order to determine their transverse or cross-breaking strength. It was found that the unimpregnated cores had an average modulus of rupture of 175 psi. The impregnated cores exhibited an average modulus of rupture of 1,825 psi. These results show that impregnation with mannitol produced an improvement in strength of over 10 times. In this example of the invention, as well as in all other examples, the impregnated cores were white and indistinguishable in appearance from the unimpregnated cores.

EXAMPLE ll Refractory cores measuring one-eighth inch by onehalf inch by 3 and 9/16 inches were made by injection molding a refractory composition containing 20 percent by weight organic plastic and plasticizers and percent by weight refractory powder. Following a low temperature heat treatment, the cores were fired at 2,200 F. for 2 hours and then cooled to room temperature. The cores were found to have an average modulus of rupture of 430 psi.

ldentically produced cores were preheated to 410 F. and impregnated for three minutes in a molten bath of mannitol at the same temperature. The impregnated cores were given a first rinse in boiling water followed by a more thorough rinse in isopropyl alcohol. The average modulus of rupture of the impregnated cores was found to be 4860 psi which is an increase of over 1 1 times that of unimpregnated cores. In addition, the impregnated cores exhibited a large increase in hardness.

EXAMPLE lll lmpregnated and unimpregnated cores produced as in Example II were placed in the die of a wax injection machine and wax patterns were injection molded around the cores. Ceramic shell molds as described in US. Pat. Re. 26,495 were produced around the patterns using a refractory slurry formed of three parts zircon powder and two parts fused silica powder suspended in a bonding liquid consisting essentially of a colloidal silica sol, a small amount of an organic film former, and minor amounts of a wetting and de-foaming agent. The first two coatings of each mold were sanded with granular zircon and each of the remaining four coatings of the molds were sanded with a coarse fire clay grog. Each dip coating was allowed to dry thoroughly before application of the next dip coating. Following the application of the sixth and final coating, each mold was allowed to dry over night. The final wall thickness of the molds was approximately three-sixteenths of an inch.

The wax patterns were removed from the shell molds in an autoclave, and the molds containing the cores were then fired at a temperature of about l,800 F. for approximately 10 minutes preparatory to casting metal into the molds. After taking the molds from the furnace, it was observed that all cores were white and that it was impossible to distinguish the cores which had been impregnated from the cores which had not been impregnated. The molds were used to produce steel castings and the castings made against the treated cores were comparable to those made against untreated cores.

EXAMPLE IV Dulcitol was melted and heated to approximately 410 F. Ceramic cores of the type used in Example [I were preheated to the same temperature and impregnated by dipping the cores into the molten dulcitol bath for minutes. Following impregnation the cores were rinsed in the manner of Example ll. The average modulus of rupture of the impregnated cores was 5,l40 psi in comparison to an average modulus of rupture of 430 psi for untreated cores.

EXAMPLE V 2-amino-2( hydroxymethyl)-l ,3 propanediol was melted and heated to 380 F. Preheated cores of the type used in Example II were impregnated by dipping for five minutes. After removal from the molten bath and after draining, the cores were rinsed quickly in warm isopropyl alcohol followed by a thorough rinse in benzene. The cores were cooled and dried to room temperature and their average modulus of rupture was found to be 4,100 psi.

Although the improvements in core strength were excellent, the compound used in this Example is not preferred because it was found not to be as stable as other compounds from the standpoint of permitting repeated melting and reuse. For example, when the impregnating agent was remelted and used in the same manner, the core strength obtained was 2,800 psi. While this strength was over six times higher than that obtained by untreated cores, it was substantially less than obtained by the first use of the material.

EXAMPLE Vl Azelaic acid was melted and heated to 260 F. Preheated cores of the type used in Example I] were impregnated by dipping for 3 minutes. Following impregnation, the cores were rinsed in hot water. After cooling and drying at room temperature, the average modulus of rupture was found to be 3,290 psi.

EXAMPLE VlI Cores similar to those used in Example ll were impregnated in molten resorcinol at 275 F. for minutes. The average modulus of rupture was found to be 2,870 psi, an increase of over six times the strength of unimpregnated cores.

EXAMPLE VllI Hydroquinone was melted and heated to a temperature in the range of from 350 to 370 F. Cores for a typical small commercial part were impregnated with the melt for periods of from 3-14 minutes. All of the impregnated cores showed a substantial increase in strength and hardness compared to unimpregnated cores. The particular cores used in this example had a stem portion nine-sixteenths of an inch in diameter and 2 inches in length. Prior to impregnation, the stem sections of the cores were easily broken between the tingers. The stem portions of impregnated cores could not be broken.

EXAMPLE lX Ceramic cores of the type used in Example II were heated to 360 F. and impregnated for 3-5 minutes in molten adipic acid at a temperature of from 360 to 380 F. After draining, the cores were rinsed quickly in boiling water and cooled to room temperature. The average modulus of rupture was 2,730 psi.

EXAMPLE X Cores of the type described in Example II were impregnated with various organic compounds having a cyclic structure, melting points greater than F., and densities greater than one gram per milliliter. In each instance, the cores were pre-heated to a temperature near the temperature of the melted impregnating material. The material used, the times and temperatures of impregnation, and the corresponding results of modulus of rupture tests are as follows:

lmpregnating lmpregnating impregnating Modulus of Agent Temp. (F) time minutes) Rupturet psi) benzil 260 5 3430 succinic anhydride 380 3 3320 anthracene 500 3 4450 benzamide 330 3-5 3820 succinimide 300 3-5 2650 EXAMPLE Xl Comparative modulus of rupture tests were conducted on cores impregnated with cyclic and noncyclic compounds having melting points above 170 F. and the same functional or substituient groups. The following table sets forth the impregnating agents, their formulas, melting points, densities, and the strengths which were obtained.

As will be apparent from the foregoing table, the strengths of the cores are not dependent upon the particular functional or substituient group of the impregnant. Each of the cyclic compounds used produced greatly increased strengths compared to non-cyclic compounds having the same groups.

Many modifications and variations of the invention will be apparent to those skilled in the art in the light of the foregoing disclosure. Therefore, it is to be understood that, within the scope of the appended claims, the invention can be practiced otherwise than as specifically described.

What is claimed is:

1. In a metal casting process in which molten metal is cast into the cavity of a mold against a refractory core, the improvement comprising the steps of:

a. strengthening and hardening said core to prevent damage during pre-casting operations, said strengthening and hardening step being carried out by at least partially impregnating said core with a hot melt of at least one organic compound selected from the classes consisting of non-cyclic hydroxyl containing compounds and cyclic compounds, said compound being characterized by:

i. a melting point of at least F.,

ii. the capability of being melted to a liquid and resolidified upon cooling,

iii. a density of at least 1 gram per milliliter,

iv. the capability of being volatilized upon heating to a temperature above the melting point;

c. and subsequently heating said core prior to casting metal into the mold to volatilize said compound from the pores of the core.

2. The improvement as claimed in claim 1 wherein said organic compound is selected from the class of non-cyclic hydroxyl containing compounds consisting of solid acids and solid alcohols.

3. The improvement as claimed in claim 1 wherein said organic compound is a solid, aliphatic compound having four to six carbon atoms and three or more hydroxyl groups.

4. The improvement as claimed in claim 3 wherein said organic compound is a straight chain polyhydric alcohol.

5. The improvement as claimed in claim 4 wherein said organic compound is selected from the group consisting of mannitol, dulcitol, sorbitol and erythritol.

6. The improvement as claimed in claim 1 including the step of heating the core prior to impregnation to a temperature approximately that of the melt. 

2. The improvement as claimed in claim 1 wherein said organic compound is selected from the class of non-cyclic hydroxyl containing compounds consisting of solid acids and solid alcohols.
 3. The improvement as claimed in claim 1 wherein said organic compound is a solid, aliphatic compound having four to six carbon atoms and three or more hydroxyl groups.
 4. The improvement as claimed in claim 3 wherein said organic compound is a straight chain polyhydric alcohol.
 5. The improvement as claimed in claim 4 Wherein said organic compound is selected from the group consisting of mannitol, dulcitol, sorbitol and erythritol.
 6. The improvement as claimed in claim 1 including the step of heating the core prior to impregnation to a temperature approximately that of the melt. 